The invention is directed to an electrolytic bath for depositing low carat, bright (glossy) gold-silver alloy coatings consisting essentially of (or consisting of) an aqueous solution of 0.5 to 25 g/l of gold in the form of alkali gold cyanide, 0.25 to 15 g/l of silver in the form of alkali silver cyanide and 10 to 200 g/l of alkali cyanide and an additive as hereinafter defined.
The electrolytic deposition of gold-silver alloy coatings has been known for many years. At current densities of 0.1 to 1 A/dm.sup.2 thereby alloy layers having a content of 29 to 79% gold can be deposited. However, with decreasing gold content and increasing layer thickness the brittleness of the coating layer increases. Besides these coatings are not very bright.
By addition of organic materials and/or foreign metals which are not co-deposited to the electrolytes there can be deposited bright coatings. For example, there are known compounds of nickel, antimony, titanium, and indium as brightening imparting additives for electrolytic gold-silver baths. For example, there are known as organic brighteners triphenylmethane dyes (East German Pat. No. 98698) and condensation products of polyalkylenimines and alkylenepolyamines (German Pat. No. 2713507). In Swiss Pat. No. 629260 there is described as a brightening imparting additive a water soluble indium compound together with an organic aliphatic amine, in Moriarity U.S. Pat. No. 4,121,982 the combination of selenium containing compound and a polyethylenimine and in German Pat. No. 1213196 the combination of a selenium containing compound and a titanate ester stabilized with an aminoalcohol. All of these known brighteners, however, have the disadvantage that a satisfactory brightness can only be obtained up to layer thickness of about 25 .mu.m and in a given case, organic materials are built into the layer. The brittleness of the coating cannot be reduced.
In the known gold-silver-alloy baths the gold content in the coating is substantially dependent upon the Au/Ag ratio in the bath and on the operating parameters chosen in the deposition, especially the current density and the bath temperature. These dependencies are disadvantageous for a practical bath operation for depositing low carat gold-silver alloys having constant carat content.